a) Field of the Invention
The present invention relates to a biodegradable mulch comprising at least two different layers of biodegradable polymer coated onto a sheet of a cellulosic material, wherein one of these layers is a layer of polylactide. More particularly, the present invention is directed to a mulch having a preprogrammed biodegradability and to methods for preparing and using the same.
b) Brief Description of the Prior Art
Bed type culture consists in harrowing the soil, then applying an agricultural mulch using a mechanical machine in such a way that mulch edges are dipped and recovered by the soil. Although not environmentally friendly, plastic mulches have been used for a long time for this type of culture. Cellulosic mulches have been experimented for many years but have never found technical acceptance. The main problem about cellulosic mulch is a premature biodegradation, particularly at the air-soil interface where biodegradation is particularly high. There is thus a need for a cellulosic mulch that is fully biodegradable but not too much in order to resist efficiently to biodegradation for an entire seasonal growth and, at the same time, capable of meeting the requirements of modern mechanized agricultural production.
It is well known in the art to coat a cellulosic sheet with a latex polymer or biodegradable polyesters. Such coated structures have found many applications, particularly in the packaging and disposable article industries and such coated structures are described in U.S. Pat. Nos. 5,163,247; 5,458,933; 5,798,436; 5,852,166; European patent application EP 514 137 and International patent application WO 98/00459. However, these coated structures have been conceived for the packaging industry and they cannot be used in the field. Indeed, the constituents of these coated structures or residues thereof are either not fully biodegradable, or either too stiff, too glossy or not sufficiently resistant to biodegradation to be used as an agricultural mulch. None of these structures can be preprogrammed to have a sufficiently useful life to serve as a mulch capable of lasting for an entire seasonal growth once deposited in the ground, nor can they be preprogrammed according to the type of soil, the type of climate nor the type of culture wherefore it is intended to be used.
The research group of the present inventors has recently disclosed in International patent application WO 99/38904 an agricultural mulch having most of the above-mentioned desired properties. This mulch comprises a sheet of paper coated with at least one layer of polylactide. As it is well known, polylactide (polylactic acid or PLA) is one of the rare polymers which are considered to be truly biodegradable. Its degradation cycle is based on a process of hydrolysis of the polymer chain. It is decomposed into lactose, water and carbon dioxide. PLA is also completely biocompatible. This means that neither the polymer nor its degradation products, are damaging to the health of living organisms. Unexpectedly, the present inventors have found that it is possible to improve the PLA coated mulch by applying to the sheet of paper at least one further layer of another biodegradable polymer. Surprisingly, the tearing strength and the stiffness of the resulting multilayer mulch is greatly increased without any detrimental effect to the biodegradation properties.
An object of the present invention is to provide a fully biodegradable multilayer mulch sufficiently resistant to biodegradation to last for an entire seasonal growth and, at the same time, sufficiently resistant and stiff to meet the requirements of modern mechanized agricultural production. The purpose of the invention is also to provide a mulch capable of being preprogrammed to have a biodegradation rate varying according to the type of soil, the type of climate and the type of culture wherefore it is intended to be used.
According to an aspect of the invention, the mulch comprises a sheet of a cellulosic material having an upper face and a lower face; and at least two different layers of biodegradable polymers coated onto at least one face of said sheet, one of these layers being a layer of polylactide. Preferably, an uppermost layer among these layers of polymers is a layer of polylactide. The mulch is capable of installation on the ground with a mechanical unrolling device, although it exhibits an increased tearing strength and an increased stiffness when compared to a mulch coated solely with a layer of polylactide.
According to another aspect of the invention, the biodegradability of the mulch is controlled by varying: a) the weight of the sheet; b) the thickness of the sheet; and/or c) the weight of the layers of biodegradable polymers coating the sheet. Preferably, the sheet of cellulosic material has a weight between about 40 to about 100 g/m2, a thickness between 60 to 175 xcexcm, and it is selected from the group consisting of virgin wood fibers, recycled wood fibers, cotton fibers and mixtures thereof.
Typically, the layers of polymers weigh from about 10 g/m2 to about 50 g/m2 and comprise from about 30 to about 70 w/w percent of polylactide. Preferably, polylactide is selected from the group consisting of D-polylactide, L-polylactide and mixtures thereof, and the biodegradable polymer(s) other than polylactide is selected from the group consisting of polycaprolactone, polyesters and mixtures thereof.
It is also an object of this invention to provide a method of preparing a biodegradable mulch comprising the steps of: providing a sheet of a cellulosic material having an upper face and a lower face; and depositing on at least one face of the sheet by a co-extrusion process at least two different layers of biodegradable polymers, wherein one of these layers is a layer of polylactide. Using such method, it is possible to deposit simultaneously the layers onto the sheet and also coat simultaneously two different cellulose sheets and thereby produce sandwich-like structure mulches.
It is still an object of this invention to provide a method of improving agricultural production comprising the steps of:
providing a biodegradable mulch comprising a sheet of a cellulosic material having at least one face coated with two different layers of biodegradable polymers, wherein one of said at least two different layers is a layer of polylactide; and
placing the mulch on the ground to provide a growth zone for plants.
Preferably, an uppermost layer among the layers is a layer of polylactide and the mulch is provided in the form of a roll and it is placed on the ground with a mechanical unrolling device. Typically, the mulch is deposited on the ground such that its layer(s) of polylactide is in contact with the soil but it can also be deposited on the ground such that the layer(s) of polylactide faces air. This method has the advantages of providing a growth zone free of weeds and also buffer soil temperature during day and night.
An advantage of the present invention is that the mulch exhibits an increased tearing strength and an increased rigidity when compared to a mulch coated solely with a layer of polylactide. The use of multiple layers of polymers also provide a better coating and accordingly, resistance to biodegradability.
Other objects and advantages of the present invention will be apparent upon reading the following non-restrictive description of several preferred embodiments, made with reference to the accompanying drawings.